The present invention relates to a spread spectrum communication system, which may be used for radio-frequency communication.
FIGS. 1A and 1B show an example of a spread spectrum communication system according to a DS (direct sequence) system in related art. FIG. 1A shows a block diagram of a transmitter thereof and FIG. 1B shows a block diagram of a receiver thereof. The transmitter comprises a pseudo noise (PN, hereinafter) signal generator 30, multipliers 32 and 33, a carrier generator 34 and a transmission antenna 35. The receiver comprises a receiving antenna 41, an inverse spreading unit, a primary demodulation unit 43, and a PN-synchronizing loop 45. (There, and hereinafter, each of devices PN synchronizing loop, phase synchronizing loop, and delay lock loop has a function of providing a reference (described below) PN signal which is produced as a result of being synchronized with a PN signal included in a received signal. Such synchronization is established as a result of circulation of an initial PN signal through the PN synchronizing loop. Such a function is similar to that of the so-called phase locked loop with regard to the basic function of keeping the phases of two signals constant relative to one another. Each of such devices as mentioned above used for realizing the present invention has a function of processing digital signals while generally speaking a phase locked loop has a function of processing analog signals.)
As shown in FIG. 1A, in the transmitter, a given data signal 31 and a PN signal provided by the PN signal generator 30 are multiplied by the multiplier 32 so that the data signal 31 is spread. Then, the spread data signal is multiplied, with a radio-frequency carrier supplied by the carrier generator 34, by means of the multiplier 33, so that the spread data signal is carried by the radio-frequency carrier and sent through the antenna 35. In the receiver, as shown in FIG. 1B, the PN synchronizing loop 45 such as a delay lock loop, for example, provides a PN signal so that the provided PN signal is synchronized with the PN signal included in the received signal received through the antenna 41. Inverse spreading is performed on the received signal so that the PN signal is removed, by means of the inverse spreading unit 42, from the received signal using the synchronized PN signal provided by the synchronizing loop 45. Then, the carrier component associated with the received signal is removed by the primary demodulation unit 43. Thus, in such an example of a spread spectrum communication system according to the DS system mentioned above, in the receiver, a primary demodulating system such as the unit 43 is necessary. Such a primary demodulating system performs functions, such as reproduction of the carrier component and detection, for example, on the signal obtained as a result of the inverse spreading operation. Thus, the construction of the receiver of such an example is rather complicated and has a rather large circuit scale, leading to a high cost.
In Japanese Patent Application 4-183255, entitled Spread-spectrum communication system, the following system has been proposed: A first correlator measures a correlation between a Manchester-code PN signal and the received signal and a second correlator detects the correlation between a non-return-to-zero-code (NRZ-code, hereinafter) PN signal and the received signal. Then, the correlation outputs provided from the first and second correlators are multiplied so that a phase error signal concerning of PN signals is obtained. The phase error signal concerns a time error between the timing of the PN signal included in the received signal and the timing of the NRZ-code and Manchester-code (reference) PN signals. Thus, a synchronizing loop is formed.